Polymer foam, thermoformed shapes thereof and method of forming same

ABSTRACT

A foam structure having a density of less than about 20 lbs/ft 3 . The method of forming the same comprising forming a homogeneous melt blend under pressure of a polymer member selected from the group consisting of polyetherimide resin particles, polycarbonate resin particles, a polymer blend of polyphenylene oxide and polystyrene in particle form imbibed with a blowing agent, followed by extrusion of the same into a lower pressure atmosphere. A composition comprising said particles imbibed with methylene chloride, chloroform, 1,1,2-trichloroethane or mixtures thereof. A method of thermoforming said foam in sheet form.

This is a division of copending application Ser. No. 560,641, filed Dec.22, 1983 U.S. Pat. No. 4,535,100.

The present invention relates to a polymer foam, thermoformed shapesthereof and methods of preparing the same.

Aircraft and other structures employ foamed polymers for insulation andstructural purposes. The electrical industry employs foamed polymers forelectrical insulation. It is essential that foams of these types becomparatively resistant to heat. The art is ever on the alert for a foamwhich is fire resistant and which gives off low levels of smoke or toxicfumes as it is heated to degradation temperatures.

One aspect of the present invention relates to a novel foam structurewhich is highly heat resistant and which has inherent flame resistancewith low smoke evolution. In another aspect, the present inventionrelates to a method of preparing such foams. In yet another aspect thepresent invention relates to thermoformed shapes of such foams.

SUMMARY OF THE INVENTION

The present invention relates to a polymer in foam form having a densityof less than about 20 lbs/ft³, wherein said polymer is selected from thegroup consisting of a solvent imbibable polyetherimide, polycarbonateand a blend of polyphenylene oxide and polystyrene, wherein said solventis a member selected from the group consisting of methylene chloride,chloroform, 1,1,2-trichloroethane and mixtures thereof.

The preferred polyetherimides imbibable with said solvent are thosewithin the following chemical structure: ##STR1## wherein Ar is adivalent organic radical containing from 6-20 carbon atoms, R is abivalent radical selected from the group consisting of aliphatic,cycloaliphatic, aromatic and araliphatic, and n is a integer having avalue greater than 1, for instance 2, 3, 4 or greater.

The preferred polycarbonates imbibable with said solvent have thefollowing chemical structure ##STR2## see Text Book of Polymer Science,2nd Edition, Fred W. Billmeyer, Jr., 1971, Wiley-Interscience, N.Y.,N.Y., page 456, the disclosure of which is incorporated herein byreference. Suitable commercially available polycarbonates are the LEXAN®polycarbonates from General Electric Company.

The preferred thermoplastic blends of polyphenylene oxide andpolystyrene are blends of poly 2,6-dimethyl-1,4-phenylene oxide and ahigh impact polystyrene.

The blend can be in a 20 to 80 weight percent ratio of either component.The term "high impact polystyrene" as used herein is intended to begeneric to both the high impact polystyrene and the high impactcopolymers derived from the isomeric methyl ethenyl benzenes mixturesand rubbery backbone polymers disclosed in U.S. Pat. No. 4,284,733, thedisclosure of which is in its entirety incorporated herein by reference.

The present invention also relates to a composition comprising discreteparticles of a polymer imbibed with a solvent member selected from thegroup consisting of methylene chloride, chloroform,1,1,2-trichloroethane and mixtures thereof and wherein said polymer is amember selected from the group consisting of a polyetherimide, apolycarbonate and a polymer blend of polyphenylene oxide andpolystyrene, said imbibed particles being in at least substantially freeflowable form, said solvent being present in a quantity sufficient tofoam said resin to a density less than about 20 lbs/ft³.

In a preferred form the foam of the present invention has a density ofless than about 5 lbs/ft³.

The present invention further relates to a process of preparing a lowdensity foam structure of the above-identified polymers comprising:

(a) providing said polymer in particle form imbibed with theabove-identified solvent.

(b) homogeneously melt blending said particles and solvent in anextrusion system under pressure;

(c) extruding said melt blend into a lower pressure atmosphere to causesaid solvent to vaporize yielding a foam structure.

During said process, the melt blend homogenizing can occur at asignificantly lower temperature because of the presence of the imbibedsolvent in the resin particles. If the resin and solvent are separatelycombined during processing, temperatures of 200° F. or higher could berequired for reasonable processing rates and then a separate coolingstep would have to be introduced into the process and system.

The present invention also relates to a method of preparing athermoformed structure of the subject foam comprising:

(a) subjecting a sheet of the polymer foam having a density of less thanabout 20 lbs/ft³ to a first temperature sufficient to permit deformationthereof;

(b) effecting a shape in said sheet while at said temperature; and

(c) reducing the temperature of said sheet to a second temperaturepermitting permanent retention of said shape at or below said secondtemperature.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be illustrated mainly with reference to thepolyetherimides.

Polyetherimides of the type contemplated by the present invention havebeen known for some time but their preparation in foam form such thatthey have a density of less than about 20 lbs. per cubic foot arebelieved to be hitherto unknown. The contemplated polyetherimides arethose which can be foamed according to the present process by meanswhich comprise the use of a solvent member selected from the groupconsisting of methylene chloride, chloroform, 1,1,2-trichloroethane andmixtures thereof. U.S. Pat. Nos. 3,787,364 and 4,024,110, thedisclosures of which are in their entirety incorporated herein byreference, disclose polyetherimides which can be solvent imbibed andfoamed according to the present process. Preferred polyetherimides arethose having the chemical structure shown above.

Polymers within the scope of this structure can be prepared byprocedures outlined in the article by D. M. White et al entitledPolyetherimides Via Nitro-Displacement Polymerization . . . ", etc.Journal of Polymer Science: Polymer Chemistry Edition, Vol. 19,1635-1658 (1981), copyright 1981, John Wiley and Sons, Inc. Particularreference is made to the preparation of polymer "(18 ip)", having amolecular weight (Mw) of about 21,000, on page 1653 thereof. Thedisclosure of this article is incorporated in its entirety herein byreference. A specific example of the preparation of anotherpolyetherimide within the contemplation of the present invention is asfollows:

EXAMPLE 1

A mixture containing 2.855 parts of 1,3-bis(4-phenoxyphthalimido)benzene, 1.180 parts of Bisphenol A, 0.081 part of o-phenylphenol sodiumsalt and 20 parts of N-methylpyrrolidone is heated to reflux undernitrogen atmosphere. The heating is continued for one hour during whichtime an approximate total of 10 parts of liquid is distilled off. Thereaction mixture is cooled and poured into about 300 parts of methanolwhich is stirred in a blender. A white polymer is precipitated. Thepolymer is filtered, washed and dried under vacuum. The polymer is apolyetherimide within the structure defined above, wherein the precursorof Ar is Bisphenol A and R is phenylene.

A commercially available polyetherimide resin which corresponds to theabove recited chemical formula is Ultem® 1000 available from GeneralElectric Company, Plastics Operations, One Plastics Avenue, Pittsfield,MA. This material has a T_(g) of 421° F. It is available in particleform having a size roughly 1/16-1/8 inch in diameter by 1/16- 3/16 inchin length. The following table details certain characteristics of theresin.

                  TABLE                                                           ______________________________________                                                           ASTM             ULTEM                                     MECHANICAL         TEST    UNITS    1000                                      ______________________________________                                        Tensile strength, yield                                                                          D638    psi      15,200                                    Tensile modulus, 1% secant                                                                       D638    psi      430,000                                   Tensile elongation, yield                                                                        D638    %        7-8                                       Tensile elongation, ultimate                                                                     D638    %        60                                        Flexural strength  D790    psi      21,000                                    Flexural modulus, tangent                                                                        D790    psi      480,000                                   Compressive strength                                                                             D695    psi      20,300                                    Compressive modulus                                                                              D695    psi      420,000                                   Gardner impact      --     in-lb    320                                       Izod impact        D256                                                       notched (1/8")             ft-lb/in 1.0                                       unnotched (1/8")           ft-lb/in 25                                        Shear strength, ultimate                                                                          --     psi      15,000                                    Rockwell hardness  D785     --      M109                                      Taber abrasion (CS 17, 1 kg)                                                                     D1044   mg wt.   10                                                                   loss/                                                                         1000                                                                          cycles                                             THERMAL                                                                       Deflection temperature, unannealed                                                               D648                                                       @264 psi (1/4")            °F.                                                                             392                                       @66 psi (1/4")             °F.                                                                             410                                       Vicat softening point, method B                                                                  D1525   °F.                                                                             426                                       Continuous service temperature                                                                    --     °F.                                                                             338                                       index (UL Bulletin 746B)                                                      Coefficient of thermal expansion                                                                 D696    in/in-°F.                                                                       3.1×10.sup.-5                       (0 to 300° F.), mold direction                                         Thermal conductivity                                                                             C177    Btu-in/  1.5                                                                  h-ft.sup.2 -°F.                             FLAMMABILITY                                                                  Oxygen index (0.060")                                                                            D2863   %        47                                        Vertical burn (UL Bulletin 94)                                                                    --      --      V-O @                                                                         0.025"                                                                        5 V @                                                                         0.075"                                    NBS smoke, flaming mode (0.060")                                                                 E662                                                       D.sub.5 @ 4 min             --      0.7                                       D.sub.MAX @ 20 min          --      30                                        ______________________________________                                    

In carrying out the present process, it is preferred to employ apolyetherimide that is anhydrous so as not to introduce the likelihoodof forming acidic components through the combination of H₂ O and thesolvent. Any such acid products would be corrosive to the extrusionequipment and possibly degradative to the polyetherimide or its foamstructure. Subjecting the Ultem® 1000 particles to a temperature ofapproximately 300° F. for a period of about 4 hours will assure at leastthe substantial absence of H₂ O in the resin.

In preparing a homogeneous melt blend of the blowing agent and thepolyetherimide, if the two materials are combined by bringing themtogether in the barrel of an extruder, as is done in the case ofpolystyrene and isopentane for example, high processing temperatures andadditional cooling equipment would be necessary in order to extrude apolyetherimide foam. In such a case the processing temperatures wouldhave to be in the range of from about 650°-700° F. and a separate highpressure control system would be necessary for the introduction of theabove-defined blowing agent. Thereafter, a separate cooling means orzone would need to be employed in the system before extrusion could takeplace otherwise the foamed system would collapse and an inferior foamstructure would result.

In order to avoid this, it has been found that the polyetherimideparticles can be readily impregnated with the above-identified class ofblowing agents and the impregnated resin particles can be melt processedat a temperature below about 500° F. and as low as from about 425°-450°F. To impregnate the Ultem® 1000 particles they merely need be subjectedto a concentrated environment of the blowing agent vapors, for example,at room temperature for a period of up to about 48 hours. This willyield free flowing particles containing sufficient blowing agent to forma low density foam. The Ultem® 1000 resin particles can contain up toabout 30 parts of blowing agent per 100 parts of resin without anyproblem of inter particle adhesion. The particles are preferablyimpregnated with 3-20 parts of blowing agent per 100 parts by weight ofresin. By employment of the blowing agent-impregnated Ultem® 1000particles this permits the use of significantly lower processingtemperatures and simpler processing equipment since the blowing agent isalready effectively dispersed throughout the resin matrix.

The following example illustrates the simplicity of the process.

EXAMPLE 2

Anhydrous Ultem® 1000 particles were exposed to an environment ofmethylene chloride at room temperature for a period of approximately 48hours to yield free flowing particles containing about 15 parts byweight methylene chloride per 100 parts of resin. These particles werefed into the feed throat of a single screw extruder having an L/D ratioof 24:1. The screw had a diameter of 11/4". The impregnated particleswere homogeneously melt blended at a temperature of 450° F. andthereafter transported directly to the die area of the system. Thehomogeneous melt was extruded through a capillary die having a diameterof 0.080 inches and a L/D of 16.1. The extruded product had a density ofapproximately 2.5 lbs/ft³ with a fine closed cell structure. It has beendetermined that during and within a short period (about 2 hours) afterthe extrusion, the methylene chloride was virtually totally expelledfrom the foam cells to yield the excellent foam product. The foam resinretains all of the high flame resistant and low smoke generationcharacteristics of the virgin polymer.

As indicated above the employment of chloroform, 1,1,2-trichloroethaneor mixtures thereof and with methylene chloride will result in anequivalent foam. It is to be understood that conventional additives,such as nucleating agents, may be added to the starting material or meltbefore extrusion.

The foam resins of the present invention can be thermoformed into anydesired shape. Generally this involves preheating a sheet of the foamstructure so as to gradually bring the temperature of the foam up tomolding temperature and thereafter shaping the foam by means of either,a female mold assisted by some force to draw the softened foam intoconformation with the mold, or by the use of matching male and femaledies.

By way of example, a polyetherimide foam sheet of a density less thanabout 20 lbs/ft³ can be formed employing the resin of Example 2 in thesystem described in Example 2, modified by the employment of a circularslit die which will yield a sheet of about 90 mils thickness. The foamsheet can be preheated to a temperature of about 475°-525° F. andincrementally advanced to male and female dies which will conform theresin sheet into a plurality of semi-circular sheaths two of which mayaccommodate the insulation of a conduit having an outside diameter ofabout 1 inch. After the structures are thermoformed in the foam sheet,the molds are cooled, the thermoformed sheet removed and the impressedstructures are separated from the selvage of the sheet.

What is claimed is:
 1. A polymer blend in foam form having a density ofless than about 20 lbs./ft³, whereby said polymer is a solvent imbibableblend of polyphenylene oxide and a high impact polystyrene, wherein saidsolvent is a member selected from the group consisting of methylenechloride, chloroform, 1,1,2-trichloroethane and mixtures thereof.
 2. Thepolymer foam of claim 1 wherein said polyphenylene oxide is poly2,6-dimethyl 1,4-phenylene oxide.
 3. The foam of claim 2 wherein saidblend is from 20 to 80 wt. % of the polyphenylene oxide and 20 to 80 wt.% of said polystyrene.
 4. The polymer foam of claim 2 having a densityof less than about 5 lbs/ft³.
 5. A thermoformed structure of apolyphenylene oxide/high impact polystyrene foam prepared by the methodcomprising:(a) subjecting a sheet of said foam having a density of lessthan 20 lbs./ft³ to a first temperature sufficient to permit a shapedformation to be effected therein; (b) effecting a shape in said sheetwhile at said temperature; and (c) reducing the temperature of saidsheet to a second temperature permitting permanent retention of saidshape in said sheet at or below said second temperature.